The present invention generally relates to an air-fuel mixture control for an automobile engine of a type having a fuel injection system and, more particularly, to the air-fuel mixture control system effective to compensate for variation in air-fuel mixing ratio of a combustible mixture by properly controlling the rate of recirculation of exhaust gases from an engine exhaust system into an engine fuel intake system in dependence on an output signal from a composition sensor for detecting the concentration of a component of exhaust gases which concentration is a function of the air-fuel mixing ratio of the combustible mixture burned in the engine.
An automobile engine of a type having a fuel injection system is well known. In this engine, an air-flow meter is installed on a fuel intake passage of the engine at a position upstream of a fuel injection nozzle with respect to the direction of flow of a combustible mixture towards the engine. The air-flow meter is used to detect, and generate an output signal indicative of, the rate of flow of incoming air which is subsequently mixed with an injected fuel to form the combustible mixture, which output signal is utilized to control the rate of supply of fuel to be injected into the engine fuel intake passage through the fuel injection nozzle. The use of the air-flow meter is advantageous in that, since the rate of supply of the fuel can be controlled in dependence on the output signal from the air-flow meter, the air-fuel mixing ratio of the combustible mixture can accurately be controlled. An example of this is disclosed in the Japanese Laid-open Patent Publication No. 53-32232 and No. 53-32233, both laid open to public inspection on Mar. 27, 1978, and corresponding to the U.S. Pat. No. 4,163,435, patented Aug. 7, 1979.
However, it has been fairly recognized that, the engine of the type having the fuel injection system requires the increased manufacturing cost because both of the fuel injection nozzle and the air-flow meter are expensive.
What appears to substantially eliminate the above described disadvantage is a control system wherein a combination of at least two of easily detectable parameters including the engine speed (number of revolution of the engine), the negative pressure inside the fuel intake passage and the opening of a throttle valve is utilized in a computer to calculate the rate of supply of incoming air so that the rate of supply of fuel which subsequently mixed with the incoming air can be controlled in correspondence with the rate of supply of the incoming air.
Where an exhaust gas recirculation system for recirculating a portion of the exhaust gases from the exhaust passage back to the intake passage for suppressing the maximum combustion temperature is provided in the engine utilizing the fuel injection system and also utilizing the air-fuel ratio control system wherein at least two of the parameters are used to compensate for variation in the air-fuel mixing ratio of the combustible mixture, such as taught by the U.S. Pat. No. 4,163,435 in view of the Japanese Laid-open Patent Publication No. 50-32327, laid open to public inspection on Mar. 29, 1975, the combustible mixture tends to be enriched because a portion of the incoming air flowing through the intake passage towards the engine is substituted by the exhaust gases recirculated through the EGR system. Therefore, in this possible arrangement, means is required to compensate for variation in the air-fuel mixing ratio which would result from the recirculation of the exhaust gases.
In order to compensate for variation in the air-fuel mixing ratio resulting from the recirculation of the exhaust gases, the conventional procedure is such as to calculate according to a control map stored in a computer, the required amount of the exhaust gases to be recirculated appropriate for a particular engine operating condition on the basis of the parameters representative of such engine operating condition, and then to control the opening of an EGR control valve disposed on an EGR passage so as to satisfy the required amount of the exhaust gases to be recirculated while a fuel adjusting means is so adjusted as to inject fuel into the intake passage at a rate or amount corresponding to the amount of the exhaust gases to be recirculated in anticipation that the exhaust gas recirculation would take place as required by the operation of the EGR control valve.
However, since this conventional method is not based on the practical amount of the exhaust gases recirculated, this conventional method involves such a disadvantage that, even though the opening of the EGR control valve is accurately map-controlled, deviation of the amount of the exhaust gases being recirculated from the required value by reason of, for example, clogging of the EGR passage results in variation of the air-fuel mixing ratio which ought to have been compensated for. Therefore, with this conventional method, an accurate control of the air-fuel mixing ratio to a stoichiometric value can not be performed.
This disadvantage may be eliminated if the amount of the exhaust gases being actually recirculated is detected to provide a reference necessary to determine the rate of supply of the fuel to be injected into the intake passage, or necessary to perform a feedback control to cause the actual amount of the exhaust gases recirculated to approximate to the required amount of the exhaust gases. However, in order to achieve this, the use of the expensive air-flow meter and/or an EGR detecting system for detecting the amount of the exhaust gases actually recirculated is required, rendering the system as a whole to be costly.
In view of the above, we have previously proposed an improved air-fuel ratio control system wherein a fuel adjusting means is provided for determining the amount of fuel to be injected by calculating from a combination of at least two of such parameters representative of an engine operating condition as the engine speed, the negative pressure and the opening of the throttle valve and wherein the air-fuel mixing ratio of the combustible mixture which has been determined by a combination of the amount of the incoming air, the amount of the fuel injected and the amount of the exhaust gases actually recirculated is detected by a composition sensor disposed on the exhaust passage, an output signal from which sensor is used to accurately control the amount of the exhaust gases to be recirculated, that is, to institute a feedback control so as to render the amount of the exhaust gases to be recirculated to be of a value approximating to the required amount of the exhaust gases.
For this purpose, the composition sensor effective to detect whether the combustible mixture burned in the engine has been enriched or leaned is utilized to detect variation in the air-fuel mixing ratio resulting from the recirculation of the exhaust gases, an output signal of said composition sensor being supplied to a computer as information so that the computer can control the EGR control valve according to such information so as to increase or decrease the amount of the exhaust gases to be recirculated in correspondence with the amount of deviation of the actual air-fuel mixing ratio from a predetermined air-fuel mixing ratio, whereby the air-fuel mixing ratio of the combustible mixture to be supplied towards the engine can be controlled to the predetermined or stoichiometric value.